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Upper Ordovician Incised Valley (Karst) Fill Deposits of Central Missouri: A Reinterpretation of “Pennsylvanian Filled Sinks”


Small, isolated bodies of sandstone completely encased within carbonate strata are scattered in an east-west trending belt along the northern flank of the Ozark uplift in central Missouri. Physical sedimentary characteristics (erosional bases and margins, graded bedding, trough cross-bedding, ripple bedding, and lateral accretion surfaces) indicate deposition by fluvial processes over a karst surface at the top of the Ordovician section. Historically, these sandstones have been interpreted as sink-hole collapse features of Pennsylvanian age based on a roughly circular geometry for many, chaotic bedding along some margins, presence of high-temperature clay minerals in associated mudstones, and stratigraphic position as the first sandstones above Ordovician carbonate formations. However, bedrock geologic mapping, outcrop architectural analyses, application of sequence stratigraphic concepts, comparison to ancient and modern analogs, and use of U-Pb detrital zircon geochronology indicate these deposits to actually be Upper Ordovician in age and to have been deposited by backfilling of narrow, steep-sided solution valleys following base-level fall and subsequent rise. This re-interpretation has important implications for timing of the tectonic and eustatic history of the Ozark region and subsequent sediment dispersal patterns. Additionally, it provides an ancient analog of lowstand “incised valley fills” in a mixed clastic/carbonate system, helping to better understand the geometry, extent, and lateral relationships of potential hydrocarbon reservoirs in such settings.